A continuing object of device performance is to increase the speed of operation. One of the techniques is the use of light as opposed to an electrical signal for the source of information for the device. The optical source needs to be converted to an electrical signal to perform a desired operation, such as controlling a computer. While different chips can be used for the optical and electrical features, it is desirable that the optical and electrical features be integrated onto one chip to decrease costs.
A photodetector is used to convert an optical signal into an electrical signal. Photodetectors can use a semiconductor material as the light absorber. Absorption of a photon can result in creation of electron-hole pairs when the photon energy is greater than the band gap of the semiconductor material. Thus the response of a semiconductor photodetector is dependent on the wavelength of the incident light. For example, the band gap of Si makes it a useful absorber, and hence photodetector, for wavelengths shorter than about 1100 nm. Important wavelengths of light used in optical communications are in bands near 1550 nm. Thus Si is not a useful material for photodetectors for these wavelengths. Ge however has a smaller band gap and can be used as a photodetector for light in both the 1310 and 1550 nm bands. However, Si remains the semiconductor of choice for the vast majority of electronic devices. Thus it would be advantageous to combine Ge photodetectors with Si integrated circuits. Furthermore since Si is transparent at the 1310 and 1550 nm wavelengths, waveguides can be fabricated in a Si chip and combined with Si integrated circuits. However, no integration method exists for forming high-quality crystalline Ge photodetectors and Si transistors on the same substrate. The integration of Ge photodetectors with CMOS integrated circuits is especially of interest because of the wide spread use of CMOS devices. While the fabrication of crystalline Ge photodetectors on Si substrates has been demonstrated, the integration of Ge photodetectors with Si CMOS integrated circuits is difficult because of conflicting materials and thermal requirements for integrating both Ge photodetectors and Si transistors on the same substrate. Therefore a need exists for forming high-quality, crystalline Ge photodetectors and Si transistors on one substrate.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.